1. Field of the Disclosure
The document relates to a display device, and more particularly, to a display device including a phase compensation film with positive wavelength dispersion.
2. Description of the Prior Art
Liquid crystal displays have advantages of light weight, thin thickness, low power consumption, and low radiation; therefore, the liquid crystal displays have replaced traditional cathode ray tube (CRT) displays of laptop computers to be widely applied to many kinds of portable electronic products in the market, such as notebooks and personal digital assistants (PDA).
A traditional vertical alignment liquid crystal display typically includes a top polarizer, a bottom polarizer, a top substrate, a bottom substrate, and a liquid crystal layer disposed between the top substrate and the bottom substrate. The top polarizer and the bottom polarizer are disposed at the outside of the top substrate and the outside of the bottom substrate respectively, and an absorption axis of the top polarizer and an absorption axis of the bottom polarizer are perpendicular to each other. When the vertical alignment liquid crystal display is a normally black mode liquid crystal display, and there is no voltage difference provided between the top substrate and the bottom substrate, the liquid crystal molecules of the liquid crystal layer do not provide different phase retardation values. Accordingly, light passing through the bottom polarizer have the same polarizing direction as the light passing through the liquid crystal layer, and the display is in a dark state. When a voltage difference is applied between the top substrate and the top substrate, the liquid crystal layer produces a half wave phase retardation, so that the polarizing direction of the light passing through the bottom polarizer is perpendicular to the absorption axis of the top polarizer because of the half wave phase retardation, and the display is in a bright state.
However, the liquid crystal molecules closer to the top substrate and the bottom substrate have larger anchoring force on the top substrate and the bottom substrate because of the rubbings of the surfaces of the top substrate and the bottom substrate. That is the top substrate and the bottom substrate may include alignment films having rubbing directions on the inner surfaces thereof. Accordingly, even in a condition of applying no voltage difference, the liquid crystal molecules closer to the surfaces of the top substrate and the bottom substrate still lie down and do not stand up. For this reason, the liquid crystal molecules lying down closer to the top substrate and the bottom substrate will affect the phase retardation value of the polarized light passing therethrough, and an observer seeing the display in a direction of a large viewing angle will see light leakage when the display is in the dark state. Also, the absorption axis of the top polarizer and the absorption axis of the bottom polarizer are not perpendicular to each other in the direction of the large viewing angle, so that the observer easily sees the light leakage, and the contrast ratio of the liquid crystal display is also affected. Furthermore, when the liquid crystal molecules are perpendicular to the top substrate or the bottom substrate, the liquid crystal molecules do not provide only one refractive index, and provides uneven refractive indexes. Since that, the light passing through the liquid crystal molecules along the direction of the large viewing angle have different phase retardation values, and the light leakage is easily generated.
Although the contrast ratio of the liquid crystal display has been improved by the phase compensation film disposed at the outside of the top substrate or the bottom substrate, the phase compensation film composed of single one material only can compensate the phase difference of the light with one wavelength. For example, when a short wavelength is 450 nm, and a long wavelength is 730 nm, the light leakage is still generated.
Besides, since the light with a shorter wavelength has larger phase retardation values while passing through the liquid crystal molecules, the material of the phase compensation film adapted to compensate the liquid crystal display generally is designed to have negative wavelength dispersion. That is to say that the light with the shorter wavelength has a smaller phase retardation values while passing through the phase compensation film with negative wavelength dispersion, so that the phase retardation values generated from the liquid crystal molecules can be compensated. However, the material with negative wavelength dispersion is not easy to be designed, and it is quite complex to manufacture this kind of material. Thus, it is not easy to use the present phase compensation film with negative wavelength dispersion to compensate the phase retardation values in all visible wavelengths, and the cost for manufacturing the liquid crystal display is easily increased largely.
Therefore, to provide a new liquid crystal display to reduce the light leakage in the direction of the large viewing angle is an objective in this field.